To capture green sea turtles in San Diego Bay, scientists pilot skiffs, set nets, throw lassos and haul in animals weighing up to a quarter-ton.

That’s the easy part.

The harder task is trying to guess the creatures’ locations — glimpsing the gliding giants, but not knowing where they’ll surface.

The large, endangered reptiles are what researcher Jeff Seminoff calls San Diego Bay’s best-kept secret, modern dinosaurs on the urban edge. Sixty or more green sea turtles live in the bay, part of the northernmost fringe of a population based in Baja California.

Scientists have tracked the creatures for decades. Until last year, the turtles could be found basking predictably at warm-water outfalls for the South Bay Power Plant. Since the facility was decommissioned two years ago and demolished early this year, the animals are venturing north of the San Diego-Coronado Bridge, confounding researchers trying to study and protect them.

With the bay restored to natural temperatures, said Seminoff, leader of the National Marine Fisheries’ Marine Turtle Ecology and Assessment Program, scientists have to discover anew “where the turtles have their bedrooms, where they sleep at night, where they forage during the day. There are still some mysteries.”

On one recent sunny morning, Seminoff and his crew set out in search of answers. Sea breezes gusted as researchers left the dock in Boston Whalers and set 328-foot long nets designed to entrap passing turtles. They also patrolled the shallows with smaller “strike nets” designed to encircle any of the turtles they might see.

Seminoff captained one boat, and database manager Dan Prosperi took the wheel of the other. Without the certainty of the power plant’s water flows, finding these reptiles was as much a matter of vibe as science, they said. It’s a kind of intuition that comes from long days at sea.

Seminoff earned his Ph.D. in wildlife ecology from the University of Arizona. But he received his field training from Mexican fishermen, whose commercial-fishing skills he adapted to scientific study. At that time, no one else in the Americas was studying green sea turtles in the water, said Bryan Wallace, chief scientist at the Oceanic Society.

Although the San Diego Bay population is small, it offers a key window into the migration and reproduction of green sea turtles throughout their range from Long Beach to Colombia, Wallace said.

“We’re really learned a lot about green sea turtles as a whole from studying this serendipitous population,” he said.

Mexico banned turtle fishing in 1990, after demand for turtle meat led the population to crash, Seminoff said. He began research in Baja California a year later and enlisted traditional fishermen’s help.

“When I first went there, I was the crazy guy that wanted to study turtles rather than eat them,” he said. “But over time, that conservation ethic I had, I was able to convey to fishermen, and they started to see the turtles outside of food.”

Under their guidance, Seminoff said, “I learned how to read the water, how to build nets from scratch, to mend nets.”

After mastering those skills, Seminoff trained others through informal apprenticeships. Among those was Tomoharu Eguchi, a marine mammal researcher who joined the turtle program eight years ago. For Eguchi, the big, placid turtles were easy to handle, compared with the feisty harbor seals and sea lions he had wrangled before. The bay, too, is milder than most.

“A big advantage of working within San Diego Bay is that weather and sea conditions are as good as they get,” he said. “This is probably the easiest work I’ve done in my career.”

Except for the waiting.

“It tests your patience,” Eguchi said.

As crews searched for sea turtles during a recent trip, several appeared underwater but slipped through gaps between the nets. Although the mesh isn’t visible, it quickly clogs with zoobotryon, invasive invertebrates that grow in clear, stringy strands. Gobs of zoobotryon alert turtles to the obstacle, allowing them to steer clear.

Despite multiple rounds of net checks, researchers found no turtles entangled that day. When the power plant was still operating, researchers could capture several animals per day. This year, they have hit the water about 10 times and caught just five, Seminoff said.

That’s not to say the turtles are gone. Last year, satellite data captured a green sea turtle named Peanut traveling north of the Sweetwater River. Later, one popped up by the USS Midway Museum and others were reported off the shore at Cardiff and Encinitas — all foreign territory for the hulking reptiles.

Although restoration of natural habitat often benefits wildlife, Wallace said the power plant demolition poses a curious irony.

“Demolishing the power plant, removing that advantage, now that means implications for growth, reproduction,” he said. “So it’s a funny reversal of the usual story. In this case, it’s removing the anthropogenic influence from the natural environment that could have some negative impacts.”

Without the warm water, turtles’ growth rates could slow. Venturing out of their protected enclave, the turtles could become more vulnerable to boat strikes and fishing-line entanglement, experts said.

Nonetheless, Wallace said, “It’s not like the turtles are all going to go away.”

Indeed, on a subsequent outing, Seminoff’s team landed a 60-pound, 26-inch long juvenile, possibly a newcomer to the bay. The presence of younger, smaller green sea turtles may mean that San Diego Bay is still attracting newcomers despite the power plant’s demolition.

After pulling in this turtle, researchers affixed a device designed to record the animal’s movements through navigational data and video footage. If Seminoff mined the centuries-old knowledge of traditional fishermen to hone his research techniques, postdoctoral student Junichi Okuyama took the best that 21st-century technology could offer for his tracking device.

Hoping to pinpoint when, where and how long the turtles eat and sleep, he used depth, acceleration and GPS data loggers to investigate their behavior.

But without the ability to see the animals, “It was impossible to know what the turtles actually did underwater,” Okuyama said.

So he added a specialized, waterproof GoPro — a miniature video camera used by athletes and movie producers — to shoot reality footage beneath the sea. The video recordings show a turtle’s eye view of the changing bay.

The return of San Diego Bay to a more natural state lets researchers study the species under new ecological conditions, providing a rare chance for comparison with the power-plant years. It’s both a scientist’s dream and a vexing logistic challenge.

“We’re in year one of a study of a natural San Diego Bay habitat,” Seminoff said. “Many scientists would die for this opportunity. Here we have this natural laboratory where that’s happening.”

Federal status: Endangered

Eastern Pacific population range: Long Beach to Colombia

Appearance: Olive-green to dark-gray or black shell with lighter underside

Size: Shell length of 28 to 43 inches, weight of 180 to 350 pounds

Life cycle: Born on nesting beaches in Baja California, live on the high seas, then settle into coastal areas and return to spawn at birth site through magnetic navigation

Life span: Reach sexual maturity in 25 years, live for 80 to 100 years